Sources for STL files and design software

Almost all CAD and 3D software can export an STL or OBJ file of the designed part – this is a file defining in a very simple/basic method how the object will look, and this file is generally read only (since it throws away everything about how it was created).

On Thingiverse you can download STL or OBJ files that someone else created (if you need the same as they needed). You can also publish your own designs.

This STL or OBJ file is what you import into software called a slicer (Cura, Slic3d, Simplify 3D are all slicers – each one have different settings that can be set and the resulting file may be slightly different, but all of these have Profiles that are compatible).
Using the correct/compatible profile, you can then have the software process your objects (the STL or OBJ files) into what is known as G-code – it is a very simple instruction file telling the printer how it should move and extrude the filament to create the object from the file.

You then place this g-code file onto the microSD card and tell the printer to print it – a few hours later you take out the object from the printer.

As to software to create the objects, it is up to you:

Low Budget:

Fusion360 have a Hobbyist and Startup license which is free. It is a cloud based package, so you need a stable Internet connection, but your objects are automatically backed up and you are always using the latest version.

MeshLab – free to use, I usually only use it to correct some pre-created files, but it can do 3d sculpting.

MeshMixer – again one I use to fix files before printing.

Blender – a Free and Open Source 3d modeling package (can also do 3d animation videos). This is very powerful especially when sculpting.

OpenSCADD – a free and Open Source CAD package where you program the objects (the format is a simple script/text file, where you define everything about the object and then tell it to Render).